FIG. 7 is a simplified representation of a xerographic micro-assembler 10 similar to that disclosed in U.S. Pat. No. 7,332,361 (“Xerographic Micro-Assembler”, which is assigned to the assignee of the present invention, and incorporated herein by reference in its entirety), and represents one type of system that performs an electrostatic assembly process. Xerographic micro-assembler 10 generally includes a sorter 40 and an image transfer unit 50. Sorter 40 includes a container 41 holding a microchip ink 20 comprising micro-objects (chiplets) 30 that are suspended in carrier solution (i.e., a mixture of non-polar liquid and charge-control agents). Sorting unit 40 further includes a conveying unit 43 having one end extending into microchip ink 20, and a second end disposed adjacent to a transfer drum 51 of image transfer unit 50. Microchip ink 20 resembles the liquid developer found in some conventional xerographic copiers and printers, but differs in that chiplets 30 (which are generally analogous to toner particles) are charge-encoded in a way that identifies each chiplet 30 and specifies its orientation such that, during the sorting process, chiplets 30 are manipulated within microchip ink 20 such that they assume associated defined positions and orientations on conveying unit 43 (e.g., as indicated by chiplets 30A in FIG. 7). That is, sorting unit 40 has the capability of electrostatically and magnetically manipulating chiplets 30 based on their individual charge encoding by way of a dynamic electrostatic field applied to conveyor member 43 by an associated controller (not shown), and to cause chiplets 30 to adhere to conveyor member 43 at designated locations and with select orientations. Chiplets 30 are then transferred in to rotating transfer drum 51, whose outer surface is processed by an image writer 53 to carry an electrostatic image that holds chiplets 30 in locations and orientations related to that established on conveyor member 43 (e.g., as indicated by chiplets 30B in FIG. 7). Chiplets 30 are then transferred from rotating transfer drum 51 onto a substrate (e.g., a printed circuit board) 60 at locations and orientations related to those established on conveyor member 43 and drum 51 (e.g., as indicated by chiplets 30C in FIG. 7). Substrate 60 is then transferred from image transfer unit 50 for post processing to interconnect chiplets 30 (e.g., by way of electrical wiring) to form the final electronic micro-assembly device.
In order to provide the necessary inks for the micro-assembler, a reliable method compatible with conventional semiconductor processing is needed for producing arbitrary charge patterns on the chip surface. A significant challenge is to design a process which protects the patterned material that provides the charge encoding. Furthermore, an ideal process and material set would be independent of the underlying device, so would be applicable to chiplets containing any functionality, such as CMOS circuitry, sensors, LEDs, etc. In addition to the charge pattern itself, the process should also provide for interconnection to underlying circuitry and singulation into small chiplets. As such, a materials set must be selected that allows for this functionality (charge pattern, interconnection and singulation) without any damage caused either to the underlying chip or the deposited layers in the stack.
What is needed is a method for producing microchip inks including microelectronic components (chiplets) that are optimized for electrostatic assembly. What is particularly needed are low-cost methods for producing charge-encoded chiplets used in electrostatic assembly.